1. Field of the Invention
The present invention relates to a remaining capacity calculating device and method for electric power storage which calculates the remaining capacity of an electric power storage such as a secondary battery and an electrochemical capacitor.
2. Related Art
These days, the electric power storages including a secondary battery such as nickel hydride battery and lithium-ion battery and an electrochemical capacitor such as electric double layer capacitor have been improved in downsizing and high energy density, and frequently used as the power source of various devices ranging from an information communication unit like a cell phone to an electric vehicle and a hybrid car.
In order to make an effective use of the electric power storage, it is important to recognize its remaining capacity accurately, and a technique of requiring the remaining capacity according to the calculation of charge/discharge currents of an electric power storage and a technique of requiring it according to an open-circuit voltage have been known.
For example, JP-A-06-242193 discloses a technique of requiring the remaining capacity at a halt time according to an open-circuit voltage obtained from a battery voltage at a stop of a electric vehicle, detecting a discharge electric capacity according to the integrated value of the discharge current of a battery, calculating the full charged amount according to this discharge electric capacity and the remaining capacity at the stop, and requiring the remaining capacity according to this full charged amount and the discharge electric capacity.
JP-A-08-179018 discloses a technique of requiring the remaining capacity according to the current integrated amount during a certain time period of charge and discharge, the voltage before discharge or charge, and the voltage after discharge or charge, in a battery having a linear proportional relation between battery capacity and battery voltage, like a lithium-ion battery.
JP-A-11-223665 discloses a technique of correcting a calculation method of the remaining capacity, according to the remaining capacity obtained by integrating the charge/discharge current of a battery and a change rate of a difference between the above remaining capacity and the remaining capacity assumed based on the open-circuit terminal voltage of the battery.
However, the technique of requiring the remaining capacity by adding up the charge/discharge currents and the technique of requiring it according to an open-circuit voltage have each good and bad points. The former is tough to load fluctuation caused by incoming current and it can get the stable remaining capacity, however, it has a drawback that there may occur accumulation of errors easily (error becomes enormous, in particular, at a time of continuous high load), while although the latter can get the accurate value in a current stable region, it has a drawback that the calculated value changes easily when a load changes rapidly and drastically.
Therefore, even a combination of the both techniques, like JP-A-06-242193, JP-A-08-179018 and JP-A-11-223665, cannot eliminate the accumulation of errors through the current integration. In a continuous charge/discharge state like a hybrid car, in particular, there is a fear that the calculation accuracy of the remaining capacity is deteriorated and the calculated value of the remaining capacity rapidly changes and it is difficult to secure the stable accuracy.